Platinum chlorine evolution reaction

Comparisons between analytical and synthetic systems are not always easy to draw in view of the different conditions employed. Thus, the thermodynamically reversible and mechanistically simplistic hydrogen and chlorine evolution reactions, well behaved in analytical studies at platinized platinum, are both beset (chlorine most severely) in large-scale chlor-alkali cells by adherence of gas bubbles to the electrode surface. 

Oxides of Platinum Metals Anodes of platinum (and more rarely of other platinum metals) are used in the laboratory for studies of oxygen and chlorine evolution and in industry for the synthesis of peroxo compounds (such as persulfuric acid, H2S2O8) and organic additive dimerization products (such as sebacic acid see Section 15.6). The selectivity of the catalyst is important for all these reactions. It governs the fraction of the current consumed for chlorine evolution relative to that consumed in oxygen evolution as a possible parallel reaction it also governs the current yields and chemical yields in synthetic electrochemical reactions. 

To summarise, AC methods have proved most successful where the system is straightforward and can be modelled analytically. By measurement over a wide range of frequencies the constants for the reaction steps constituting the model can be established and, particularly if adsorbed species are involved, AC methods have proved very powerful indeed, with a major area of application being in the study of metal passivation, as discussed in detail elsewhere in the book. An example of this behaviour in practice is provided by the work of Conway s and Hillman s groups on chlorine evolution at platinum. Several mechanisms for this reaction have been proposed, including both Volmer and Heyrovsky types ... 

Chlorohydrins and dichloroalkanes are formed in electrogenerative chlorination of olefins on platinum black (50). The former predominate at high currents and low anode potentials, with no side reactions of chlorides, such as chlorine evolution. From the identified products, the following tentative mechanism has been proposed for ethylene chlorination (50) ... 

Since the oxygen overvoltage on a platinum electrode is equal to 0.45 V, the minimum potential required for oxygen evolution is (+ 0.813 + 0.45) V or 1.263 V. Since the voltage required to implement this reaction is lower than that for a normal chlorine electrode, it follows that oxygen will be evolved at the anode in preference to chlorine. In contrast to the two possible reactions at the anode, there are five reactions that are possible at the cathode as shown below ... 

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